This invention relates to vibratory feeders, conveyors, separators, screens, and other vibratory equipment used in materials handling and related fields (hereinafter referred to generally as vibrators) in which the motive force is provided by an imbalance generator of the type having unbalanced weights on a rotating shaft.
A number of methods are used to obtain a straight line motion of the driven or working member of a vibrator. One known method uses electromagnetic motors employing an electromagnet with an air gap. Part of the electromagnet is usually in the form of an "E" and is fitted with an electrical coil or coils, and the other part of the magnetic circuit is in the shape of an "I" fitted to the other part of the vibrator with an air gap between them, so that the two may vibrate relative to one another. Examples of vibrators using electromagnetic vibratory motors are described in U.S. Pat. Nos. 2,163,249; 2,654,466 and 2,694,156.
Electromagnetic vibratory motors of this electromagnetic type are restricted to vibrators having a short stroke and, since the air gap is in the direction of motion of the moving parts, is subject to drift in size and striking of the two parts of the magnetic circuit due to expansion or contraction caused by temperature variation, by fluctuations in supply voltage, or by changes in the natural frequency of vibration of the resonant system. On the other hand this type of motor does not generate forces in undesired directions which have to be decoupled.
Where larger stroke amplitudes are required in a two-mass tuned vibrator it is usual to generate the vibratory forces by an imbalance generator of the type having unbalanced weights on a rotating shaft. The shaft may be driven by any suitable means, such as a belt drive, but most commonly the shaft is part of the armature of an electric motor with eccentric weights secured to each end of the armature shaft. This imbalance generator, which constitutes one mass of the two-mass system, is coupled to the second mass, the work member, by resilient means. The second mass or work member is suitably isolated from the ground so that it may be regarded as a free mass.
The rotating unbalanced weights produce oscillatory forces which may be resolved into two components, a desired force in the direction of the desired vibratory motion, and an unwanted force at right angles thereto.
The rotational angular velocity of the imbalance generator is selected so that the desired oscillatory force has a frequency which is close to the natural frequency of the two mass system as a whole, typically 0.85 of the said natural frequency.
In order to decouple the unwanted oscillatory force from the work member the natural resonant frequency of the two mass system in the direction of the unwanted oscillatory force must be small, of the order of one quarter the frequency of the unwanted force, and this requires that the resilient coupling means have a low effective spring rate in the direction of this force.
Satisfactory decoupling has been achieved in vibrators of the above type by employing rubber or like polymer materials as the resilient means between the imbalance generator and the working member, such resilient means having different spring rates in the two directions. Rubber and polymer springs are affected by heat, oil, aging, ozone attack, and other adverse conditions, requiring frequent replacement.
Helical coil springs can be used for coupling the two masses. An arrangement of this type is described in U.S. Pat. No. 3,348,664 in which a single spring is used between the imbalance generator or motor and the working member or trough. The spring rate in the desired direction (hereinafter called the axial direction) is required to be at least twice the spring rate in the direction of the unwanted force (hereinafter called the lateral direction). In practice spring rate ratios of four or more are desirable, but it is not possible to obtain such a ratio with a helical coil spring if the spring ends are held parallel. An effective spring rate ratio of four to one can be obtained if the spring is allowed to flex in the lateral direction. However in the arrangement of U.S. Pat. No. 3,348,664, the spring is retained rigidly at its ends by clamps, resulting in the spring being subject to both tensile and compressive forces, and severe clamping strains are imposed on the ends of the spring. Any spring clamped in this way must be severely derated, since the maximum stress acts on the clamp and can be more than double the stress that the spring would be able to withstand if used entirely in compression. Frequent failure of the spring results due to stress fracturing adjacent to the mounting. Also the isolation of the vibratory movement in the lateral direction is poor, making for an elliptical motion of the working member.
A spring system for coupling the two masses has been described in U.S. Pat. No. 3,583,246 using opposingly operating pre-loaded springs which are inserted between two spring supports. It is an essential requirement of the vibrator described in this specification that the centre of gravity of the imbalance generator be at least approximately in the medial plane between the spring supports, while a characteristic of the arrangement is its small structural height, which is assisted by the use of a number of short coil springs having a few turns only. This arrangement requires that, if the spring ends are to deflect sideways with respect to each other, the spring ends must remain parallel and, while the spring ends remain parallel, a large spring rate, similar in value to the axial spring rate, is obtained. Accordingly the lateral natural frequency of this arrangement is close to the axial natural frequency, and decoupling is poor.